1. Field of the Invention
This invention relates to improvements in a focus detecting apparatus for automatically detecting the position of the point of focus of the photo-taking lens or other lens of a photographic camera, a video camera or the like by the positional relation of a secondary image.
2. Description of the Related Background Art
In a focus detecting apparatus in which a secondary imaging optical system is disposed near a position equivalent to the predetermined imaging surface of a main imaging lens such as a photo-taking lens, the image of an object formed near the position equivalent to the predetermined imaging surface is further separated into a plurality of secondary images. The plurality of secondary images are received by a plurality of light-receiving means and the positional relation between the plurality of secondary images is found to thereby obtain the position of the point of focus of the main imaging lens (the defocus amount from the predetermined imaging surface). Such an apparatus is well known, for example, from U.S. Pat. Nos. 4,559,446 and 4,699,493, etc.
FIGS. 8A and 8B of the accompanying drawings show the construction of said focus detecting apparatus. As shown in FIG. 8A, the pupil 1 of a main imaging lens is imaged on a predetermined imaging surface 2 if the lens is in focus. As shown in FIG. 8B, divided pupils 3 and 4 are projected onto light-receiving elements 7 and 8 by a field lens 5 disposed near (a position equivalent to) the predetermined imaging surface 2 and a secondary imaging lens 6. When the main imaging lens is out of focus, the positional relation between the secondary images on the light-receiving elements 7 and 8 differs from a reference positional relation. Therefore, the position of the point of focus (the defocus amount) can be calculated on the basis of the deviation between the phases of the image signals of the light-receiving elements 7 and 8.
Now, in the above-described focus detecting apparatus, the accurate detection of the position of the point of focus is established where the main imaging lens is an ideal aberrationless lens, where there are aberrations in the main imaging lens, a difference occurs between the calculated position of the point of focus and the best position of the point of focus of the actual main imaging lens. Even if the calculated position of the point of focus is brought into coincidence with the predetermined imaging surface 2, defocus will occur more or less. That is, in the case of a main imaging lens in which spherical aberration is not completely corrected as shown in FIG. 9 of the accompanying drawings, the spherical aberration corresponding to the pupil 1 is a and the spherical aberration corresponding to the divided pupil 3 or 4 is b and therefore, the position of the point of focus at which the contrast is best differs.
Also, in a main imaging lens wherein correction of chromatic aberration is not complete, there may occur a case where the position of the point of focus differs. FIG. 10 of the accompanying drawings is a spectral characteristic graph showing the reason for the focus difference in such a case. In FIG. 10, the abscissa represents the position of the optical axis, and the ordinate represents the amount of light imaged at each position on the optical axis. Curve c is a characteristic curve weighted from the characteristic of a photosensitive material used for ordinary photographic lenses, and d indicates the best position of the point of focus. Curve e is a characteristic curve weighted from the spectral sensitivity characteristic of the light-receiving element used in the focus detecting apparatus, and f indicates the position of the point of focus of the received image. Generally, the characteristic curve c and the characteristic curve e do not coincide with each other and therefore, there occurs a point-of-focus difference g.
As can be seen from the foregoing description, between an ideal main imaging lens and the actual main imaging lens, there occurs a detection error of the position of the point of focus (the defocus amount) in conformity with the remaining amount of aberration.
To overcome the above-noted disadvantage, correction of the aberrations of the main imaging lens can be effected until said detection error becomes sufficiently small, but a much higher degree of correction than that required for the quality of image of a photograph or video is necessary with the result that the apparatus becomes bulky and expensive.
If the setting of the reference position of the secondary image on the light-receiving element is effected by the actual main imaging lens, the detection error will become null in that lens or at the zoom position thereof. However, when interchange of the lens is done or zooming is effected, a detection error will occur. Also, as regards chromatic aberration, correction thereof can be made by the use of a light-receiving element having the same sensitivity as the film sensitivity or by a filter or the like. Such a proposition has already been made, but it is not preferable because in the natural world, the amount of light on the infrared side is great and reception of much infrared light makes photographing in the dark advantageous and also increases accuracy.